25ft midcabin boat. From I/O to twin outboard conversion

Discussion in 'Boat Design' started by Forsmanni, Aug 31, 2020.

  1. tpenfield
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    tpenfield Senior Member

    Any update on this? I know it has been a few weeks.

    Outboard motors are a 'thing' now and many boat manufacturers that have made I/O powered boats in the past are also offering them in an outboard powered configuration. So, there are some current examples to view & review in terms of the mechanical/engineering aspects, and the power and performance metrics.

    Essentially, the outboard powered version of traditional I/O powered boats are modified by the addition of a transom bracket which provides the mounting for the outboard(s) and the additional buoyancy needed at the stern for while the boat is at rest or below planing speeds.

    The existing designed-in swim platforms of the boats are still accessible, yet significantly less so, mostly from the sides. So, far that aspect of the re-engineering has not been figured out well - IMO.
     
  2. tpenfield
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    tpenfield Senior Member

    Here is some data of I/O vs. OB configurations for a popular large bowrider (Formula 310BR) just to get an idea of power/speed/weight comparisons of I/O vs. OB configurations.

    Formula 310BR

    I/O 2 x 300HP (600 total) - 50 MPH max speed - 11,220 lbs. weight as tested

    I/O 2 x 430HP (860 total) - 59 MPH max speed - 11,120 lbs. weight as tested

    OB 2 x 350HP (700 total) - 49 MPH max speed - 11,900 lbs. weight as tested

    I would think the difference of 700-800 lbs. additional weight of the OB powered version is mostly in the bracket, which is additive.
     
    Last edited: Sep 22, 2020
  3. Forsmanni
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    Forsmanni Junior Member

    I have done a lot of research and talked to people who have made a similar conversion. I have come to the conclusion that: It doesnt make sense to go with twin carburated 2-stroke engines if you think about fuel consumption.
    So I have decided to go with one 250-300hp 2-stroke EFI outboard (4-stroke is not option). The setback that transom bracket will make and better efficiency with outboard compared to I/O should get me better handling, faster top-speed and better fuel consumption when driving on plane.
    I will have to reinforce the transom and I will make the transom bracket myself (fiber glass) so it is not a small project but I like bigger ones :)
    The 2-stroke EFI engine may be also more expensive than the EFI V8 I/O but I will get that money back from service cost and hopefully from fuel consumption.

    I am very exited about this project and Im trying to find 250hp O/B because originally the V8 was also 250hp. Then I can make good comparison between I/O and O/B.
    I will make videos to Youtube of this project and next summer I will also make head to head comparison video of this outboard conversion boat vs same boat with original carb. V8 inboard.

    The boat is Four Winns 245 Vista
    Youtube channel is BCB-garage
    Please follow and subscribe if this project sounds interesting. I have few other projects ongoing as well.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The direct injection type of EFI 2 stroke ( Optimax, for example) are not much different to EFI 4 stroke as regards fuel use, that seems like a good choice.
     
  5. fallguy
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    fallguy Senior Member

    The boat should be lighter and longer with the ob on bracket. I'd be surprised if you don't get a performance improvement is engine outputs are equal, etc.

    i know a guy who built a glass bracket and he wasn't sure it was all that much cheaper than a purchased one
     
  6. brendan gardam
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    brendan gardam Senior Member

    A 200 hp outboard will give you more speed and holeshot than a 250 hp mercruiser v8.
     
  7. tpenfield
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    tpenfield Senior Member

    I assume it is an older boat (1980's-90's) ? When you have the engine out and before you start working on the bracket installation, best to check the structure for rot. Easy to repair while the engine is out and before you rig up the OB.
     
  8. DCockey
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    DCockey Senior Member

    Why is a 4-stroke outboard not an option?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Budgetary, probably.
     
  10. Forsmanni
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    Forsmanni Junior Member

    Fallguy: Thats true. I have materials already and this isnt first bracket that Im making. My opinion is that it its much cheaper to make it yourself. We have also very limited selection here about brackets...

    brendan gardam: Yes it could be that 200hp O/B would give same top speed than 250hp I/O but the torque figures are so much smaller--> I think 250hp will give much smoother ride when going on plane and also better fuel economy at cruising speeds.

    tpenfield: Yes the boat is 1990. I have checked the transom and Im pretty sure its solid, but i will reinforce it anyway. The sides of the boat also have foam enclosures-> Im gonna remove the foam even if its not wet.

    DCockey: I just dont like 4-stroke outboards. Well Mercury R-series are awesome but I can't afford to even look towards them.
    2-stroke is lighter, sounds much better, cheaper and easier to service. Also the 2-stroke torque that 4-stroke dont have is needed in this heavy boat.
     
  11. brendan gardam
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    brendan gardam Senior Member

    I think you are way off with 4 stroke outboards. They are more robust and reliable than di 2 strokes. A drop of water can destroy a modern 2 stroke. Even though I love 2 strokes . Weights between 2 and 4 these days are getting closer. Suzuki or yamaha 250"s are great motors. I would choose them over a merc anyday.
     
  12. tpenfield
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    tpenfield Senior Member

    post some pictures and sketches as you go :)
     
  13. Forsmanni
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    Forsmanni Junior Member

    So here is first sketch of my transom bracket. 70cm setback & 70cm (27") wide box that has about 140 liters of under water volume = Lift for the stern when the boat is still. When driving on plane the bracket doesnt touch water at all. My biggest concern is how this flat bottom bracket is going to behave when driving downwind. Any thoughts or improvement suggestions?

    Capture.JPG
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I think the flat bottom is not the best way to go with the pod, you need to have it veed similar to the boat. Is the transom going to be reinforced, to take the pod ? How is the pod intended to be attached to the boat ? Can you find another identical hull that has been podded, to learn from ?
     

  15. Forsmanni
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    Forsmanni Junior Member

    I cant figure any major reason why pod bottom should be veed. I know that it would increase underwater volume and vee-shape would support the bracket little bit better but do you have some other reasons? And yes ofc Im going to reinforce the transom.
     
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